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Never had an improvement in mpg with higher octane. The engine should not knock at specified 87 octane. Knock is caused by high pressures and/or temperatures, which under normal conditions should not exist in our engines. Also, too lean mixture can cause knock. Wide open throttle at low rpm can contribute to knock by putting a large charge into the cylinder, and a slow piston speed can cause a higher pressure as the charge burns.

Normal (non-track) driving almost never involves more than 50% throttle, so the conditions for knock just aren't there. The exceptions would be when stomping on the gas when the light turns green or accelerating onto the highway.

10% is a very significant difference. Theoretically, the best fuel economy is with the lowest octane which doesn't knock. I suspect any mpg difference is due to everything other than octane.
 

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Knocks are not a constant encounter on properly operating engine, because of ECU will adjust timing after a few miles and the engine will not knock, but it will run with slightly retarded ignition timing on lower octane gas that leads to less power that leads to lower efficiency. Do not take it a literally as bad - the efficiency is still within specs. Newer DI engines are high compression ones and for sure they have a band to adjust/adapt to fuel octane variations. 89 octane keeps it at the best efficiency, because of ECU never needs to retard ignition timing, because of it never experience the knock in the first place.
 

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We needed to change over to premium. The car(Crosstrek) had horrible detonation quality , performance, and mpg on 87. Having said that, it could be the PNW petrol(poor quality, etc...). Up in BC(Canada) we could get 94 octane without ethanol and it made a huge improvement to the mpg and performance compared to the standard 91 with ethanol. Each to their own and what works for them.
regards
 

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There have been lots of threads on this topic. I think the consensus is that, all else being equal, there's no advantage to putting premium in a car tuned for 87, you're just wasting money. It was one of the things we liked about the Crosstrek, not having to put premium in the Lexus any more. Over three years now and and never any knocking or rough running (and we have Ethanol in the gas here and special formulas to control smog.
 
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2018 Crosstrek Premium, Sunshine Orange
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the consensus is that, all else being equal, there's no advantage to putting premium in a car tuned for 87, you're just wasting money
The one thing that may make a difference is if the higher octane gas is ethanol free because ethanol has less energy than a comparable volume of gasoline. See Alternative Fuels Data Center: Ethanol Fuel Basics. Occasionally I find ethanol free 89 octane at Costco for <3% more than the cost of 87 octane regular. I will pay that for the slight increase in mileage that the ethanol free gives. When the difference is greater I choose not to waste my money.
 

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Knocks are not a constant encounter on properly operating engine, because of ECU will adjust timing after a few miles and the engine will not knock, but it will run with slightly retarded ignition timing on lower octane gas that leads to less power that leads to lower efficiency. Do not take it a literally as bad - the efficiency is still within specs. Newer DI engines are high compression ones and for sure they have a band to adjust/adapt to fuel octane variations. 89 octane keeps it at the best efficiency, because of ECU never needs to retard ignition timing, because of it never experience the knock in the first place.
At lower throttle, say 75% or less as a rough guess, your engine should not knock at all. This assumes a reasonable rpm, say 2000+, and no hot spots inside the cylinder due to carbon deposits or spark plug problem, and normal use (not track day). If there is no knock for the knock sensor to sense, the ecu has no reason to retard timing. So your 87 octane will be treated by the engine the same as a higher octane by running the normal ignition timing. Running higher octane will not improve efficiency.

Tracking, towing uphill in the summer, and other non-typical uses can easily cause knocking even with higher octanes.
 

Has Butch tires
2018 Orange Subaru Crosstrek Limited
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This subject has been visited before and can be found on the archives. Consumer Reports did a study on this:
I wonder if folks here have any concrete knowledge of how the Subaru engine will identify higher octane gasoline and what changes it will make to improve the efficiency of the engine.
I have used Premium gas when towing for about 200 miles in the mountains and on the next trip using Regular. While both trips were nearly exactly the same length over the same roads I can't say that conditions (temperature, wind, etc.) were exactly the same. In any case the fuel economy was within 1 mpg and the energy when using the Premium wasn't perceptibly different so I now use Regular. Not really scientific, but it seems to agree with the Consumer Reports article.
 

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This has to be the first time in any N/A automotive forum that anyone has asked about the benefits of premium gas... :LOL:

That having been said. Not to add fuel to fire, but Subaru of Australia does say 87 is OK, higher octane is recommended. I pinged them asking why, but never got a reply. I suspect though (hasn't been corroborated by any measured data) that, with such a high compression and high IATs (>40C / 100 F), higher octane will marginally benefit (avance) ignition timing.

Now, whether the few degrees / few extra lbs.ft of torque in specific low-RPM, WOT lugging conditions are worth the increased fuel cost, that's a whole other story,,,
 

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That having been said. Not to add fuel to fire, but Subaru of Australia does say 87 is OK, higher octane is recommended. I pinged them asking why, but never got a reply. I suspect though (hasn't been corroborated by any measured data) that, with such a high compression and high IATs (>40C / 100 F), higher octane will marginally benefit (avance) ignition timing.
Where did you see that? Do you have a link?
 

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Where did you see that? Do you have a link?
Still looking for it, I could swear I saw it somewhere. I fo have the quote though (pertaining to the 2.0): "The use of unleaded fuel with an octane rating of 95 or higher is recommended "
@Montanan - yes, 95 RON is 91 AKI here for us North Americans...
 

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We needed to change over to premium. The car(Crosstrek) had horrible detonation quality , performance, and mpg on 87. Having said that, it could be the PNW petrol(poor quality, etc...). Up in BC(Canada) we could get 94 octane without ethanol and it made a huge improvement to the mpg and performance compared to the standard 91 with ethanol. Each to their own and what works for them.
regards
I live in the PNW - Central Oregon - and the 87 E10 works perfectly fine in my car, up and down to Mt. Bachelor, 30-32mpg (you'll have to do the math to liters/100km!).
 

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The one thing that may make a difference is if the higher octane gas is ethanol free because ethanol has less energy than a comparable volume of gasoline. See Alternative Fuels Data Center: Ethanol Fuel Basics. Occasionally I find ethanol free 89 octane at Costco for <3% more than the cost of 87 octane regular. I will pay that for the slight increase in mileage that the ethanol free gives. When the difference is greater I choose not to waste my money.
87 vs. 87 E10 is only about 3% more energy per gallon. So if it costs about 3% more, you're getting exactly what you pay for (and 3% more range per tank).
 

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I live in the PNW - Central Oregon - and the 87 E10 works perfectly fine in my car, up and down to Mt. Bachelor, 30-32mpg (you'll have to do the math to liters/100km!).
It was horrid in the families last Outback as well. There was so much debris in the petrol, it clogged the pump assembly at 35k(IE not stall at intersections when you come to a stop). Frankly, I think the Crosstrek is on the same timeline for replacement.
regards
 

Has Butch tires
2018 Orange Subaru Crosstrek Limited
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This has to be the first time in any N/A automotive forum that anyone has asked about the benefits of premium gas... :LOL:

That having been said. Not to add fuel to fire, but Subaru of Australia does say 87 is OK, higher octane is recommended. I pinged them asking why, but never got a reply. I suspect though (hasn't been corroborated by any measured data) that, with such a high compression and high IATs (>40C / 100 F), higher octane will marginally benefit (avance) ignition timing.

Now, whether the few degrees / few extra lbs.ft of torque in specific low-RPM, WOT lugging conditions are worth the increased fuel cost, that's a whole other story,,,
This subject has come up before. Do you have any data on how flexible the Crosstrek computer is in responding to higher octane gas? Do we know that the Crosstrek can identify and change it's timing to use the higher octane? Thanks. I can't tell sometimes how much we know and how much we think we know.
 

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I haven't seen any raw data, so at this point it's speculation/educated guesses. I'd love to see a timing advance plot of 87 vs 89 vs 91 at WOT, but alas, haven't found one.
What we do know is that the CR of the engine is quite high. We also know that in order to maximize fuel efficiency/reduce fuel consumption, stock timing is likely fairly aggressive, hovering near the knock point. At low RPMs, hight IATs and at WOT (easy to try when you have a MT), I would speculate that the knock sensor routinely retards timing. The recommendation (if confirmed) of Subaru of Australia to use higher octane gas, correlated with the fact it's a heat-soak prone country, would seem to confirm this.

As a result, I'm quite convinced higher octane gas does indeed provide some benefit. Whether it's actually measurable, tangible, and worth it, is another story. First, even a few additional degrees in timing won't shave much from your 0-60. These will be marginal gains. Second, the gains are likely to occur only near knock situations, - at WOT low RPM, and in summer - providing no benefits whatsoever in normal driving, unless you're regularly in the 40C/100F outdoor temperatures range.
 

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This has to be the first time in any N/A automotive forum that anyone has asked about the benefits of premium gas... :LOL:
I鈥檝e been on a fair amount of auto forums over the years different manufacturers different models etc. this comes up time to time on all of them regardless of drivetrain NA, turdblow, superdupercharged, low/high compression, carburetor, MPFI, SFI, GDI, bone stock, tuned, full mods, etc. even some discussions on lawn equipment, motorcycle, & ATV forums. lots of back & forth on using ethanol, altitude, seasons of the year, etc also with all of them.

my favorite threads to read are the ones where the manufacturer requires premium fuel & guys swear up & down you only need to run 87... pay 40 grand + for a 鈥減erformance鈥 or 鈥渟port鈥 vehicle & act like a extra dime or two a gallon @ the pump is going to break them. usually the same guys telling you if you don鈥檛 use Mobil1 or AmsOil with some matching filter your junk will blow up...

it would be interesting to see the fuel & timing maps from the ecm with different fuels in the Trek. guys with sister trucks to my 1/2 ton N/A GDI running the same tune/mods as mine make more 20hp on a dyno running 91/93 octane over same mods stock tune on 87. timing map curves look almost identical with a couple more degrees across the board with 91 of course. running same tune on GOOD E85 nets additional couple hp with a E85 tune same mods 40hp over stock tune same mods. stock tune stock truck 87 vs 91/93 minimum if any gains but the 91/93 timing map is noticeably less retarted.

What we do know is that the CR of the engine is quite high.
seems common across most/all manufacturers now due to the increased efficiency of DI. I have to guess being able to bump up compression was one of the deciding factors to switch to DI? will be interesting to see what happens in the next decade (before we all get stuck with electric) now that some manufacturers are experiencing carbon build up problems & some like Ford are now running 鈥渄uel鈥 fuel systems to combat it.
 
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